Method for producing iron carbide from granulated sponge iron

ABSTRACT

Iron carbide (Fe 3 C) is produced from granulated j sponge iron which has a maximum carbon content of 2 wt. % and is supplied from an iron ore reducing plant. The sponge iron is swirled with a gas containing carbon and a maximum 1.5 vol. % water in a fluidized bed reactor at temperatures ranging, from 500 to 800° C. A product is extracted from the reactor. The total iron content of said product is fixed at a minimum of 80 wt. % Fe 3 C. Preferably, methane or a gas containing methane is fed into the fluidized bed reactor as a gas containing carbon. A waste gas can be extracted from the top area of the fluidized bed reactor and hydrogen can be separated therefrom and at least partially directed into the reducing plant.

[0001] This invention relates to a process of producing iron carbide(Fe₃C) from granular sponge iron, which comes from an iron ore reductionplant with a carbon content of not more than 2 wt-%.

[0002] From the U.S. Pat. Nos. 5,527,379 and 5,603,748 the directreduction of iron oxide is known, where in several fluidized bedsgranular, iron-oxide-containing material is brought in direct contactwith hot reduction gas at temperatures of 500 to 900° C. When thereduction gas not only contains hydrogen, but also a considerablecontent of carbon monoxide, a product rich in Fe₃C can be withdrawn fromthe last stage of the fluidized bed of the known reduction process.Practice has shown, however, that in the reduction of iron oxide to ironthe steam produced greatly impedes the simultaneous formation of ironcarbide as a result of the reaction of iron with Co and/or CH₄.

[0003] It is therefore the object underlying the invention to omit thesimultaneous production of iron carbide (Fe₃C) during the directreduction of iron oxide and the formation of sponge iron. In accordancewith the invention iron carbide is produced by means of theabove-mentioned process such that the sponge iron, which has a carboncontent of not more than 2 wt-%, is swirled in a fluidized-bed reactorat temperatures of 500 to 800° C. with a carbonaceous gas whose watercontent is not more than 1.5 vol-%, and that from the reactor a productis withdrawn, whose total iron content is bound as Fe₃C for at least 80wt-%. Preferably, at least 85 wt-% of the total iron content of theproduct withdrawn are bound as Fe₃C.

[0004] In the process in accordance with the invention, carburizing thelow-carbon sponge iron is deliberately effected separate from thereduction plant. This requires a more complex apparatus than the knownproduction of iron carbide, but the reduction plant is relievedconsiderably in the process in accordance with the invention. Now, thereduction plant is preferably operated with hydrogen-rich gas asreduction gas, which contains only little CO or is virtually free fromCO. During the carburization a H₂-containing gas is produced, and thishydrogen can advantageously be utilized upon separation in the reductionplant. It is recommended to form the reduction gas supplied to thefluidized bed of the last reduction stage from hydrogen for at least 80vol-% (calculated free from nitrogen). Then, the granular sponge iron,which is supplied to the fluidized-bed reactor for carburization,usually has a carbon content of not more than 1 wt-%.

[0005] For carburizing in the fluidized-bed reactor gases rich inhydrocarbon are used, which in the reactor may also serve as fluidizinggases. As gas rich in hydrocarbon there might for instance be usedmethane or methane-containing natural gas. To accelerate thecarburization, the fluidized-bed reactor is operated at pressures in therange from 3 to 10 bar. In the fluidized-bed reactor, the solids mayform a stationary fluidized-bed, or they may be held in the state of thecirculating fluidized bed. In the latter case, the reactor comprises aseparator for the separation of solids, which is connected with theupper part of the reactor, and from which separated solids arerecirculated to the lower part of the reactor. Per hour, at least 5times the weight of solids is recirculated, as compared with the solidscontent in the reactor.

[0006] Embodiments of the process will now be explained with referenceto the drawing. It represents a flow diagram of the process.

[0007] From granular iron oxide, which is supplied via line 1, there isfirst of all produced by means of reduction sponge iron with a carboncontent of not more than 2 wt-% and preferably not more than 1 wt-%. Thereduction may be effected in any manner known per se. An advantageousprocedure is described in the already mentioned U.S. Pat. Nos. 5,527,379and 5,603,748, where a drying and heating stage 2 is followed by a firstreduction stage 3 and a subsequent second reduction stage 4. In bothreduction stages the reduction is effected in a fluidized bed, wherehot, hydrogen-containing gas is used as reduction and fluidizing gas.The temperatures in both stages 3 and 4 lie in the range from 500 to900° C. The first stage 3 is designed as circulating fluidized bed, towhich at least in part used, H₂-containing reduction gas from the secondstage 4 is supplied through line 5. The exhaust gas of the first stageis recirculated via line 6 to a processing plant 7, in which there isalso produced fresh gas rich in hydrogen. The gas is supplied as hotreduction gas through line 8 to the second reduction stage 4, in whichthe solids preferably form a stationary fluidized bed. Preferably, thegas of line 8 comprises at least 80 vol-%, and mostly at least 90 vol-%hydrogen. A partial stream of the reduction gas of line 8 is expedientlysupplied directly to the first stage 3 through line 8 a. The degree ofmetallization of the partly reduced ore of line 3 a is about 50 to 80%.

[0008] From the second reduction stage, granular sponge iron with acarbon content of not more than 2 wt-% and preferably not more than 1wt-% is withdrawn via line 10. This sponge iron is charged into afluidized bed reactor 11, which is connected with a cyclone separator12. For carburizing the sponge iron, gas rich in hydrocarbon, which forinstance chiefly consists of methane, is supplied through line 13. Thisgas first of all flows into a distribution chamber 14 and then asfluidizing gas through a tuyere bottom 15 upwards through the reactor11. In the reactor 11 the temperatures lie in the range from 500 to 800°C. A gas-solids suspension is supplied from the upper part of thereactor 11 through the passage 16 into the separator 12, and separatedsolids are recirculated to the reactor 11 through line 17. The productwithdrawn through the passage 18 from the lower part of the reactor 11chiefly consists of iron carbide, where at least 80 wt-% of the totaliron content are bound as Fe₃C. This product is supplied to a coolingunit not represented.

[0009] Solids-containing gas leaves the separator 12 through line 20 andfirst gives off heat in the heat exchanger 21. Due to the carburization,the gas of line 20 has a considerable hydrogen content, so that the H₂content, calculated anhydrous, will be at least 10 vol-%. Expediently,there should be ensured a H₂ content in the gas of line 20 of 15 to 40vol-% (calculated dry). For dedusting, the gas is first supplied to afilter 23 through line 22, and is then supplied to a wet scrubbing unit25 via line 24. In the scrubbing unit 25 washing solution is sprayed inthrough line 26, and used, solids-containing washing solution iswithdrawn via line 27. Cleaned gas is sucked in via line 29 by means ofthe blower 30. It is very advantageous to at least partly separate thehydrogen content of the gas and utilize the same in the reduction plant.For this purpose, the gas is wholly or partly supplied through line 31to a separating means 32 for separating a gas fraction rich in H₂ fromthe gas mixture. If desired, a partial stream of the gas coming from theblower 30 may be guided past the separating means 32 through the bypassline 33 and the opened valve 34.

[0010] The separating means 32 may operate in a manner known per se, forinstance according to the principle of pressure-swing adsorption, or maybe designed as membrane separation. Furthermore, it is possible toeffect a gas separation by means of deep cooling. In addition to a gasfraction rich in H₂, which is discharged via line 36, a residual gas isobtained from the separating means 32, which residual gas is withdrawnvia line 37. When a purge gas is used, such as in the case of apressure-swing adsorption plant, the latter is withdrawn via line 38indicated in broken lines. The residual gas of line 37 is mixed with thegas of line 33 and fortified by gas rich in hydrocarbon, e.g. methane,from line 38 a, The gas mixture, which serves as carburizing gas, ispassed via line 37 a for heating purposes first through the heatexchanger 21 and then through the fired heater 40, before it is fed intothe reactor 11 through line 13.

[0011] When the H₂-rich gas of line 36 is already suited as reductiongas, it may directly be admixed to the reduction gas of line 8.Otherwise, this gas is supplied to the processing plant 7 through line36 a indicated in broken lines.

EXAMPLE

[0012] A procedure corresponding to the drawing involves the productionof 600 000 t Fe₃C per year from an iron ore with an Fe content of 67wt-%, which comprises 96 wt-% Fe₂O₃. In addition to other gangue, theore contains 2.4 wt-% SiO₂. The data have been calculated in part. Inthe first reduction stage 3, which is designed as circulating fluidizedbed, the solids temperature is 630° C., and in the stationary fluidizedbed of the second reduction stage 4 the temperature is 640° C.

[0013] The reduction gas of line 8 comprises 90 vol-% H₂ and stillcontains 9 vol-% N2 and 1 vol-% H₂O .

[0014] The sponge iron of line 10 has a content of metallic Fe of 88.0wt-%, an FeO content of 8.5 wt-%, and it contains 3.5 wt-% SiO₂; thecarbon content is negligibly small.

[0015] For carburizing, sponge iron is supplied to the reactor 11 in anamount of 63.6 t per hour, the pressure in the reactor is about 4 bar,the temperature is 600° C. In the various lines, the following gasquantities flow, and their temperatures and their components CH₄, H₂ andH₂O are indicated in the table. Line 13 20 31 33 36 37 38a Gas 128,400135,700 49,800 85,500 14,300 33,600 9,300 quantity (Nm³/h) Tempera- 730600 85 60    40 40   25 ture (° C.) CH₄ 79.2 69.6 69.6 69.6 — 98.7   100(vol-%) H₂ 19.8 29.5 29.5 29.5   100 — — (vol-%) H₂O 1 0.9 0.9 0.9 — 1.3— (vol-%)

[0016] The separating means (32) is a pressure-swing adsorption plant,and 9300 Nm³/h CH₄ are withdrawn through line 38 a. The productwithdrawn from the reactor 11 comprises 89 wt-% Fe₃C, 8 wt-% FeO and 3wt-% Sio₂.

1. A process of producing iron carbide (Fe₃C) from granular sponge iron,which comes from an iron-ore reduction plant with a carbon content ofnot more than 2 wt-%, characterized in that in a fluidized-bed reactorat temperatures of 500 to 800° C. the sponge iron is swirled with acarbonaceous gas whose water content is not more than 1.5 vol-%, andthat from the reactor a product is withdrawn, whose total iron contentis bound as Fe₃C for at least 80 wt-%.
 2. The process as claimed inclaim 1, characterized in that the granular sponge iron with a carboncontent of not more than 1 wt-% is supplied to the fluidized-bedreactor.
 3. The process as claimed in claim 1 or 2, characterized inthat methane or methane-containing gas is supplied to the fluidized-bedreactor as carbonaceous gas.
 4. The process as claimed in any of claims1 to 3, characterized in that from the upper part of the fluidized-bedreactor solids-containing exhaust gas is withdrawn, solids are separatedfrom the exhaust gas, and separated solids and exhaust gas are at leastpartly recirculated to the reactor.
 5. The process as claimed in claim4, characterized in that from the exhaust gas withdrawn hydrogen isseparated, which is introduced into the reduction plant at least inpart, before the exhaust gas is recirculated to the fluidized-bedreactor at least in part.
 6. The process as claimed in claim 1 or any ofthe preceding claims, characterized in that the pressure in thefluidized-bed reactor is 3 to 10 bar.
 7. The process as claimed in claim1 or any of the preceding claims, characterized in that the granularsponge iron comes from a reduction plant, in which it is treated as alast reduction stage in a fluidized bed by supplying hydrogen-containingreduction gas at temperatures in the range from 500 to 800° C.
 8. Theprocess as claimed in claim 7, characterized in that the reduction gassupplied to the fluidized bed as a last reduction stage comprises atleast 80 vol-% hydrogen (calculated free from nitrogen).